The evaporation of materials for depositing thin layers is a widespread process, e.g., in technical physics and in semiconductor technology. When using materials that pass into the gaseous phase only at extremely high temperatures (e.g., above 2000° C.), there are special requirements for the evaporation technology. At first, the evaporator must be sufficiently temperature-stable in order to heat the material to be evaporated to the desired evaporation temperature. Secondly, a precise and reproducible adjustment of an evaporation rate is frequently rendered difficult due to the high energy currents necessary for achieving the evaporation temperature. Finally, there is a tendency at the high temperatures to reactions of the material to be evaporated that can be desired in the case of reactive evaporation but must be avoided as regards any impurities present.
There are special challenges in the production of thin-layer components for CMOS technology. High-melting ceramic materials, e.g., rare-earth oxides are increasingly used for forming insulating layers in the course of rising integration density. A reactive evaporation of ceramic materials in which the desired stoichiometric composition is adjusted in the insulating layer by a partial oxygen pressure during the evaporation is normally excluded in CMOS technology since undesired oxide layers are formed by the oxygen on free silicon surfaces of the thin-layer component. There is therefore interest in a non-reactive vapor deposition in which the material to be evaporated already has the desired stoichiometric composition of the insulating layer.
The following problems in the evaporation or sublimation of stoichiometric material to be evaporated are known from practice. A first problem occurs if the material to be evaporated is unstable in the ambient atmosphere. This is the case, e.g., for the ceramic Pr2O3, that decomposes under the influence of water vapor into a stoichiometrically changed oxidation state that for its part liberates undesired oxygen during the evaporation. Thus, the evaporator can only be charged at great expense with a very pure material that is unstable in the atmosphere. If a stabilizing additive substance is added to the material to be evaporated, a purification for eliminating the additive substance would be required before the evaporation of the material to be evaporated that is, however, excluded with the conventional evaporator technology for the following reasons.
The temperatures required for evaporating high-melting materials are typically produced with electron beam evaporators. In general, there is the problem in the direct heating of the material to be evaporated by an electron beam that only the surface of the material to be evaporated, is heated with the electron beam so that an inhomogeneous heating of the material to be evaporated results. If the electron beam is directed onto the material to be evaporated a strong local surface heating up to evaporation takes place but no uniform heating of the material to be evaporated. Therefore, an electron beam evaporator is not suitable for sublimation purification of the material to be evaporated.
Similar problems result from the fact that the ceramic materials of interest, e.g., for CMOS technology are frequently present as granulates or powder that render a reliable charging of evaporators difficult. The charging of the evaporator with sintered bodies of the material to be evaporated is desired in particular for large-scale industrial applications. However, the sintering process also requires the addition of a binding agent that would have to be removed before the evaporation by a sublimation purification.
It is also possible to counter the cited stability problem in that an oxidation state of the material to be evaporated that is stable in the atmosphere is charged into the evaporator and is converted before the evaporation into the desired stoichiometric oxidation state by a tempering. For example, the ceramic material Pr6O11, that is stable in air, is available in high purity but requires a tempering for conversion into Pr2O3. A homogeneous tempering with conventional electron beam evaporators is not possible on account of the local surface heating of the material to be evaporated.
It can be noticed in general that the temperature of the material to be evaporated can not be adjusted sufficiently precisely in a reproducible manner with a conventional electron beam evaporator in order to achieve a certain purification effect or tempering effect or to adjust a certain evaporation rate. This problem cannot be solved with a conventional thermal resistance heating of a crucible with a heating resistor and a temperature control with a thermocouple since in this case there is the disadvantage that the conventional resistance heating evaporators are only designed for temperatures below 1900° C.
The cited problems in the evaporation of high-melting materials exist not only in CMOS semiconductor technology but also in other evaporation tasks for purposes of coating or of purification.
Another general problem of conventional evaporation technology for high-melting materials consists in that the evaporator characteristic of an electron beam evaporator can be adjusted to be sufficiently stable only in a limited manner. The inhomogeneous surface heating of the material to be evaporated brings about an irregular change of the surface of the material to be evaporated during the evaporation and with it a change of the evaporator characteristic. However, it is essential for applications in thin-layer technology to be able to control the homogeneity of layer separation on a substrate during the entire coating procedure. For this reason, there is interest in using effusion cells for adjusting a certain vapor beam of the material to be evaporated. However, effusion cells for high-melting materials, especially for large-scale industrial purposes, have not yet been available.